(i) Field of the Invention
The present invention relates to a method for reducing the amount of a carbon contamination particularly in an interface between an epitaxial film and an Si substrate in an Si epitaxial growth or an Si.sub.1-x Ge.sub.x epitaxial growth onto the Si substrate by using a gas such as SiH.sub.4, Si.sub.2 H.sub.6, GeH.sub.4 or B.sub.2 H.sub.6 as a material and using a CVD device.
(ii) Description of the Prior Art
With regard to the technique of an Si epitaxial growth or an Si.sub.1-x Ge.sub.x epitaxial growth onto an Si substrate by using a gas such as SiH.sub.4, Si.sub.2 H.sub.6, GeH.sub.4 or B.sub.2 H.sub.6 as a material and using an ultra high vacuum type CVD device (hereinafter referred to as "the UHV-CVD device") in which vacuum exhaust is possible up to an ultravacuum region, its application to a channel epistructure in a fine CMOS at a level of 0.1 .mu.m and a next generation bipolar transistor is particularly expected. Heretofore, this kind of epitaxial growth has been carried out as follows.
In the first place, a natural oxide film on the surface of an Si substrate is removed by a dilute hydrofluoric acid (DHF) treatment, and an agent solution was then washed out with water, immediately followed by washing with an ammonia-hydrogen peroxide-pure water mixed agent solution (APM washing), to remove particles and organic substances on the surface of the substrate. Afterward, the agent solution is washed out with water. In order to remove the natural oxide film formed by the APM washing, the dilute hydrofluoric acid (DHF) treatment is further carried out. Afterward, water washing is done, and the Si substrate is then dried, for example, by the use of a spin drier. Next, the Si substrate is loaded into a growth chamber 9 of a UHV-CVD device shown in FIG. 6. The loaded Si substrate 8 is supported by a suscepter 12, and then heated by a substrate heater 11. A heater chamber 10 in which the substrate heater 11 is installed, and the growth chamber 9 are differentially exhausted by the Si substrate 8 itself, and the respective chambers are exhausted to a vacuum level of 10.sup.-9 to 10.sup.-10 Torr by a turbo-molecular pump 13. Here, a final wet pretreatment of the Si substrate is the DHF treatment, and therefore the surface of the Si substrate remains exposed. In consequence, organic substances in a clean room atmosphere are deposited on the Si substrate during an interval until it is loaded into the UHV-CVD device Applied Physics A39, p. 73 (1986)!. If the epitaxial growth is carried out, allowing the organic substances (carbon) to remain, a carbon contamination remains in an epitaxial film-Si substrate interface, so that the crystallinity of the epitaxial film noticeably deteriorates and, for example, the deterioration of electric properties such as the backward bias leak current of a pn junction occurs. Thus, after the Si substrate 8 is loaded into the growth chamber 9, it is once heated up to high temperature such as 850.degree. C. by the substrate heater 11 and then subjected to hydrogen annealing at this temperature at a vacuum degree of 10 Torr. By this treatment, the above carbon contamination can be removed (The 42nd Applied Physics Related Combination Lecture (1995) Preliminary Manuscript, 29a-Q-8). Afterward, the temperature is lowered to 800.degree. C. or less, and an Si epitaxial growth or an Si.sub.1-x Ge.sub.x epitaxial growth is then carried out onto the Si substrate 8 by the use of a gas such as SiH.sub.4, Si.sub.2 H.sub.6, GeH.sub.4 or B.sub.2 H.sub.6 as a material at a temperature of 600 to 800.degree. C. For example, in the case that a selected Si.sub.1-x Ge.sub.x epitaxial film is applied onto a base of a high-speed bipolar transistor, an Si.sub.2 H.sub.6 gas or a GeH.sub.4 gas is first used to grow a non-doped selected Si.sub.1-x Ge.sub.x epitaxial film, and an Si.sub.2 H.sub.6 gas, a GeH.sub.4 gas or a B.sub.2 H.sub.6 gas is then used to grow an in-situ B-doped selected Si.sub.1-x Ge.sub.x epitaxial film (so that aparasitic barrier by an Si.sub.1-x Ge.sub.x --Si hetero junction may not be formed at a base-collector junction).
When the Si epitaxial growth or the Si.sub.1-x Ge.sub.x epitaxial growth is carried out in the above conventional manner, a contamination, particularly boron (B) in the growth chamber 9 is deposited on the Si substrate during the hydrogen annealing which is carried out immediately before the epitaxial growth. This takes place for the following reason. In the UHV-CVD device, the B.sub.2 H.sub.6 gas or the like is used for the sake of the in-site boron doping into the epitaxial film, but when this gas is introduced into the growth chamber, boron adheres to the inside wall of the growth chamber and it remains thereon. Furthermore, also in the case that the Si substrate already doped with boron at a high concentration is treated, boron vaporizes from the surface of the Si substrate, and it adheres to the inside wall of the growth chamber and remains thereon. Then, when the temperature of the growth chamber becomes higher temperature than 800.degree. C., the remaining boron tends to volatilize from the inside wall of the growth chamber, and it is also deposited on the Si substrate.
If the temperature is lowered, allowing boron to adhere onto the Si substrate and this substrate is then subjected to the epitaxial growth as it is, boron remains in the epitaxial film-Si substrate interface. This has a bad influence on electric properties of devices such as the fluctuation of a threshold voltage in a fine CMOS (because in an n-channel transistor, the threshold voltage is controlled by a boron concentration in a channel section) and the deterioration of a cut-off frequency in a high-speed bipolar transistor using the Si.sub.1-x Ge.sub.x epitaxial film as a base (because a parasitic barrier by boron is formed at a base-collector junction).